Control of Gene ExpressionChapter 11

11.1 Proteins interacting w/ DNA turn Prokaryotic genes on or off in response to environmental changes

Gene Regulation: turning on and off of genes

Gene Expression: overall process of information flow from genes to proteins

The control of gene expression allows cells to produce specific kinds of proteins when and where they are needed

Our earlier understanding of gene control came from the study of E. Coli

A cluster of genes with related functions, along with control sequences, is called an operon With few exceptions, operons exist only in Prokaryotes

11.1

When an E. Coli encounters Lactose, all enzymes needed for its metabolism are made at once using the Lactose operon

The Lactose (lac) operon includes

1) Three adjacent lactose-utilization genes

2) A promoter sequence where RNA polymerase binds and initiates transcription of all 3 lactose genes and

3) An operator sequence where a repressor can bind and block RNA polymerase action

11.1 Regulation of the Lac operon

A regulatory gene, located outside the operon, codes for a repressor protein

In the absence of lactose, the repressor binds to the operator and prevents RNA polymerase action

Lactose inactivates the repressor, so

The operator is unblocked

RNA Polymerase can bind to the promoter

All 3 genes of the operon are transcribed

11.1

Repressor: binds and blocks RNA polymerase action

There are 2 types of repressor-controlled operons In the Lac Operon, the repressor is

Active when alone

Inactive when bound to Lactose

In the trp bacterial operon, the repressor is

Inactive when alone

Active when bound to the amino acid Tryptophan (Trp)

Another type of operon control involves activators, proteins that turn operons on by Binding to DNA and

Making it easier for RNA polymerase to bind to the promoter

Activators help control a wide variety of operons

11.1B

11.1C

11.2 Chromosome Structure & Chemical Modifications can affect Gene Expression

Differentiation Involves cell specialization, in structure and function, and

Is controlled by turning specific sets of genes on or off

Almost all of the cells in an organism contain an identical genome

The differences btwn cell types are Not due to the presence of different genes but instead

Due to selective gene expression

Eukaryotic chromosomes undergo multiple levels of folding and coiling, called DNA packaging

11.2

Chemical modification of DNA bases or histone proteins can result in epigenetic inheritance

Certain enzymes add a methyl group to DNA bases, without changing the sequence of the bases

Individual genes are usually more methylated in cells in which the genes are not expressed. Once methylated, genes usually stay that way through successive cell divisions in an individual

Removal of the extra methyl groups can turn on some of these genes

Inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance. These modifications can be reversed.

11.2A

11.2B

11.3 Complex Assemblies of Proteins Control Eukaryotic Transcription

Eukaryotic RNA polymerase requires the assistance of proteins called Transcription Factors.

Transcription Factors include Activator proteins, which bind to DNA sequences called

enhancers and initiate gene transcription. The binding of the activators leads to bending of the DNA

Other transcription factor proteins interact with the bound activators, which then collectively bind as a complex at the gene’s promoter

RNA polymerase then attends to the promoter and transcription begins

Silencers are repressor proteins that May bind to DNA sequences

And inhibit transcription

11.3

11.4 Eukaryotic RNA may be spliced in more than one way

Alternative RNA splicing

Produces different mRNAs from the same transcript

Results in the production of more than one polypeptide from the same gene and

11.4

11.5 Small RNAs play multiple roles in controlling gene expression

MicroRNAs (miRNAs) can bind to complementary sequences on mRNA molecules either Degrading the target mRNA or

Blocking its translation

RNA Interference (RNAi) is the use of miRNA to artificially control gene expression by injecting miRNAs into a cell to turn off a specific gene sequence

11.5

11.6 Later Stages of Gene Expression are also subject to regulation

After mRNA is fully processed and transported to cytoplasm, gene expression can still be regulated by

Breakdown of mRNA

Initiation of translation

Protein activation

Protein breakdown

11.7 -Review